Hydrocarbon synthesis (HCS) methods utilizing Fischer-Tropsch processes are well known and described in the art. In a Fischer-Tropsch process, synthesis gas (CO+H2) made, e.g. from natural gas, is converted over a catalyst, e.g. a ruthenium, iron, or cobalt catalyst, to form a wide range of products including gaseous and liquid hydrocarbons, oxygenates and a normally solid, high paraffin hydrocarbon wax. Typically, Fischer-Tropsch waxes are upgraded by catalytically converting them to lower boiling paraffinic hydrocarbons falling within the gasoline and middle distillate boiling ranges. This treatment primarily involves hydrogenation, e.g. hydroisomerization, hydrocracking, hydrorefining and the more severe hydrorefining referred to as hydrotreating. However, as new markets expand, the demand for high quality waxes as end products has increased. The varied and growing uses for high quality Fischer-Tropsch waxes include e.g. food containers, waxed paper, coating materials, electrical insulators, candles, crayons, markers, cosmetics, etc. Stringent purity requirements that a wax must meet are set by regulatory authorities such as the FDA in the United States and the SCF in the European Union, particularly if the wax is to be used in food and drug applications.
Fischer-Tropsch waxes have many desirable properties. They have high paraffin contents and are essentially free of the sulfur, nitrogen and aromatic impurities found in petroleum waxes. However, untreated raw Fischer-Tropsch waxes may contain small but significant quantities of olefins and oxygenates (e.g. long chain primary alcohols, acids and esters) formed in the slurry as by products of the HCS reaction. Consequently, there is a need to further treat raw Fischer-Tropsch wax to remove these impurities. This additional treatment is part of a time consuming and costly process as Fischer-Tropsch waxes typically undergo hydroprocessing in order to achieve high purity. These purification measures typically occur in another reactor separate from the reactor where the hydrocarbon synthesis has occurred. In addition, different catalysts are used to hydroprocess the wax. Accordingly, there is a need for a more efficient and direct method of producing purified Fischer-Tropsch wax from a hydrocarbon synthesis process.
A preferred process mode for operating the Fischer-Tropsch process is a slurry-type process which may be carried out, e.g. in moving bed systems or slurry reactors. The slurry comprises slurry liquid and finally divided catalyst, wherein the catalyst particles are suspended in a liquid hydrocarbon and the CO/hydrogen mixture is forced through the catalyst/hydrocarbon slurry allowing good contact between the CO/hydrogen and the catalyst to initiate and maintain the hydrocarbon synthesis process.
Advantages of a slurry-type process, over that of a fixed bed process are that there is better control of the exothermic heat produced in the Fischer-Tropsch process during the reaction and better control over catalyst activity maintenance by allowing recycle, recovery, and rejuvenation procedures to be implemented. The slurry process can be operated in a batch or in a continuous cycle, and in the continuous cycle, the entire slurry can be circulated in the system allowing for better control of the primary products' residence time in the reaction zone.
Slurry reactors, sometimes referred to as “bubble columns,” are well known for carrying out highly exothermic, three phase slurry-type Fischer-Tropsch reactions. As disclosed in U.S. Pat. No. 5,348,982, in a three-phase hydrocarbon synthesis (HCS) process, a synthesis gas comprising a mixture of H2 and CO (syngas) is bubbled up as a third, gaseous phase through the slurry in the reactor. The slurry comprises liquid hydrocarbons and dispersed solid particles comprising a suitable Fischer-Tropsch type hydrocarbon synthesis catalyst. The catalyst particles are typically kept dispersed and suspended in the liquid by the lifting action of the syngas bubbling up through the slurry and by hydraulic means. Typically, the slurry liquid is the product of the reaction, usually C5-C100 hydrocarbons. Preferably, the slurry liquid comprises primarily high boiling paraffins (Fischer-Tropsch waxes).